1. Field of the Invention
The present invention relates to an optical attenuator, and more particularly, to a variable optical attenuator for optical communication.
2. Related Art
In optical communication, e.g., in wavelength-division multiplexing (WDM) transmission, a variable optical attenuator is used to adjust the intensity level of light to an appropriate range.
FIG. 8 shows an example of the variable optical attenuator, which comprises a shutter plate 21. The shutter plate 21 is located for movement in a direction (X-direction) across the optical path of an optical fiber 3. As the shutter plate 21 moves, the amount of light obscuration changes. This arrangement is described in IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 1, January/February 1999, pp. 18-25.
As mentioned before, some variable optical attenuators are used for WDM transmission. The variable optical attenuators for WDM transmission are expected to be able to achieve optical attenuation of 30 dB or more, suffer only a small polarization-dependent loss, stand high optical input power, and be small-sized.
It is hard for the variable optical attenuators shown in FIG. 8 to meet the above requirements for the following reason.
In the variable optical attenuator shown in FIG. 8, a diffraction pattern that is formed on the light receiving end face of the optical fiber spreads over an area wider than the fiber end face, owing to diffraction by means of the shutter plate 21. Since the reflectance of light incident upon the outer peripheral edge portion of the optical fiber end face varies depending on the direction of polarization, furthermore, a polarization-dependent loss is caused.